The real reason liberals accuse Tea Partiers of racism is that contemporary American-style liberalism is in rigor mortis. Liberals have nothing else to say or do. Accusations of racism are their last resort.
The European debt crisis — first Greece, then Portugal and now Spain (and Belgium, Ireland and Italy, evidently) — has shown the welfare state to be an unsustainable economic system.

He goes on:

Liberals, leftists or progressives — whatever they choose to call themselves — have a great deal of trouble accepting this. To do so they would have to question a host of positions they have not examined for years, if ever, not to mention have to engage in discussions that could threaten their livelihood and jeopardize their personal and family associations.
Thus the traditional wish to kill the messenger who brings the bad news: the Tea Partiers.

Others are saying similar things, but Roger's piece explains it especially clearly and covers a lot of territory succinctly. I recommend the whole thing.
Hat tip: John at Powerline